The limitations and affordances of holographic video displays are chiefly dictated by the spatial light modulators upon which they are built. The temporal bandwidth of the spatial light modulator determines the display size, view angle, and frame rate. The pixel pitch determines the angle of the display or the power of the lenses needed to achieve a wide view angle. The space-bandwidth product, which is related to the numerical aperture of the holographic grating, determines the maximum depth range and number of resolvable views the display will possess. Finally, optical non-idealities of the modulator give rise to noise and artifacts in the display output.
Current state-of-the-art technologies for spatial light modulation (e.g., liquid crystal (LC), micro-electro-mechanical systems (MEMS) [Kreis, T., Aswendt, P., and Hofling, R., “Hologram reconstruction using a digital micromirror device,” Optical Engineering, vol. 40, pp. 926-933 (2001); Pearson, E., “MEMS spatial light modulator for holographic displays”, S. M. Thesis, Massachusetts Institute of Technology (2001)], and bulk-wave acousto-optic modulators [Hilaire, P., Benton, S., and Lucente, M., “Synthetic aperture holography: a novel approach to three-dimensional displays,” Journal of the Optical Society of America A, vol. 9, pp. 1969-1977 (1992)]) have proven challenging to employ in holographic video displays. The currently-employed modulators are challenging to use for several reasons, including low bandwidth (relative to holograms), high cost, low diffraction angle, poor scalability, quantization error, and the presence of zero and other order noise, unwanted diffractive orders, and zero-order light, as well as spatial or temporal multiplexing of color. These issues must therefore be addressed before using such modulators in a holographic display system.
Much of the cost and complexity of modern holographic displays is due to efforts to compensate for these deficiencies by, for example, adding eye tracking to deal with low diffraction angle [Haussler, R., Schwerdtner, A., and Leister, N., “Large holographic displays as an alternative to stereoscopic displays,” Proceedings of SPIE Stereoscopic Displays and Applications XIX, p. 68030M (2008)], duplicating and phase shifting the optical path to eliminate the zero order [Chen, G.-L., Lin, C.-Y., Kuo, M.-K., and Chang, C.-C., “Numerical suppression of zero-order image in digital holography.” Optics Express, vol. 15, pp. 8851-8856 (2007)], or creating large arrays of spatial light modulators to increase the display size [Sato, K., A. Sugita, M. Morimoto, and K. Fujii, “Reconstruction of Color Images at High Quality by a Holographic Display”, Proc. SPIE Practical Holography XX, p. 6136 (2006)]. The cost and complexity of holographic video displays can be greatly reduced if a spatial light modulator could be made to have better affordances than the LC and MEMS devices currently employed.
Full-color, video-rate holograms stereograms using arrays of waveguides with acoustic grating patterns that diffract in one axis only (horizontal parallax only or HPO) have previously been produced [D. Smalley, Q. Smithwick, V. M. Bove, Jr., J. Barabas, S. Jolly, “Anisotropic leaky-mode modulator for holographic video displays.” Nature 498.7454, pp. 313-317 (2013); D. Smalley, Q. Smithwick, J. Barabas, V. M. Bove, Jr., S. Jolly, and C DellaSilva, “Holovideo for everyone: a low-cost holovideo monitor,” Proc. 9th International Symposium on Display Holography (ISDH 2012) (2012)]. The advantages of polarization rotation, enlarged angular diffraction, and wavelength division for red, green, and blue light have further been demonstrated.